The present invention is concerned with the abatement of noise generated by axial fans. The term "axial fan" is used here to refer to any fan of the general type in which the flow of air or other gas is in a direction parallel to the axis about which the fan blades rotate. This class of fans includes the common propeller fans in which there is no shroud as well as tube axial fans which are located in a shroud, usually a duct. Tube axial fans are often of the vane axial type in which stator vanes are employed to suppress rotational flow in a downstream direction.
There are three known basic techniques for suppressing the noise generated by axial fans. One technique is to position noise absorptive material in regions near the fan or otherwise adjacent to the fluid flow. While this technique is helpful, its effectiveness is limited, particularly in certain frequency ranges. If the absorptive material is positioned in such a way that it does not interrupt the fluid flow, thus minimizing back pressure and increasing fan efficiency, its noise reducing ability is further diminished.
The second known noise abatement technique entails positioning side branch resonators along the path of the fluid flow, particularly in the case of tube axial fans. These resonators, in essence, reflect noise of a particular frequency band causing it to be dissipated in large measure before it escapes from the tube or duct. A properly designed side branch resonator can be highly effective in removing spikes. For example, where there is often a noise spike at the fan blade passage frequency which can be attenuated in this way.
Conventional side band resonators are mounted on the outside of the duct. In many environments, however, there is little or no room available in which to mount a resonator at the desired location. This problem is frequently encountered, for example, in mine shafts where noise abatement is a chronic problem. Although a conventional resonator might be placed inside the duct, it would in this way tend to significantly reduce the cross-sectional area of the flow path and might, therefore, produce more noise than it would remove.
The third known noise abatement technique focuses on the fact that air flow from an axial fan generally follows a path, in the region near the fan, that is of a generally annular configuration, since the fan has a central structure in which its motor is usually housed that blocks flow along the axial center of the path. This creates a central region of noise producing turbulence behind the fan and may produce a similar region in front of the fan.
To eliminate the noise from this source, a center body is sometimes positioned near the fan, on either side or on both sides, to fill the center volume that lies outside the main flow path, thus reducing or eliminating the turbulence. If properly designed, center bodies are effective in reducing noise, although they generally do not eliminate all such turbulence associated noise, partly because there is turbulence between the fan and the center body and because there are discontinuities in the cross section of the flow path attributable to the very presence of the center body itself.
It is a primary objective of the present invention to provide an improved noise abatement apparatus and axial fan combination.